add edge-pod-network doc

docs/tutorial/edge-pod-network.md

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+In edge scenario, edge node can be offline since edge network is unstable, 
+thanks to node autonomy feature provided by yurt-hub, pods can still be working,
+however there are some cases we need to take care.
+
+### Flannel: keep Vtep mac address
+If we use flannel as CNI plugin, in VXLAN mode, a Vtep device (usually named as "flannel.1") is created on each host,
+and the VNI, VtepMAC info are recorded in kube node annotation, consumed by other nodes to be aware and apply
+the corresponding routes and forwarding rules.
+Each time node restarted, flannel will create Vtep device with new mac address, and update the info in node annotation.
+
+However, if node is offline, the update cannot be truly done.
+This will cause pod connectivity issues between edge nodes as other nodes are not aware of the mac address change.
+
+**Solution: Keep Vtep mac address in flannel**
+
+For each time flannel restarted and try to create new Vtep device, it reads the mac address from node annotation, 
+either from kube-apiserver or yurt-hub local cache, and use it for Vtep device if exists.
+
+For this solution, there is minor code change in flannel.
+
+### IPAM: keep pod ip address
+In most cases, host-local IPAM is used for CNI plugin. It works by assigning free ip address for each new pod from node's CIDR range,
+and recording the assigned ips in local path, to denote them occupied.
+For some edge application, static pod ip is required, which means pod ip address cannot be changed when it's recreated.
+
+**Solution: Keep ip address in IPAM**
+
+To support this, we need to adjust the IPAM plugin to also record {ip}-{pod name} binding info, and pass pod name in request to IPAM.
+For incoming ip allocation request, IPAM use the binded ip if pod name already exists in local path.